Intermediate precursor compositions used to make supported catalysts having a controlled coordination structure and methods for preparing such compositions

ABSTRACT

Intermediate precursor compositions for use in manufacturing supported reactive catalysts having a controlled coordination structure, and methods for manufacturing such precursor compositions are disclosed. The precursor compositions include a catalyst complex formed from catalyst atoms and a control agent that is applied to a substrate. Reduction of the catalyst complex yields supported reactive catalyst in which a preponderance of the top or outer layer of atoms of the catalyst particles exhibit a controlled coordination number of 2. The supported catalysts are useful for a variety of chemical reactions, including the preparation of hydrogen peroxide with high selectivity.

BACKGROUND OF THE INVENTION

1. The Field of the Invention

The present invention relates to catalysts for use in various chemicalprocesses. More particularly, the invention relates to intermediateprecursor compositions used to make supported catalysts having acontrolled coordination structure and methods of manufacturing suchintermediate precursor compositions.

2. The Relevant Technology

Catalysts are widely used in many industries including chemical,petroleum, pharmaceutical, energy, and automotive. Many of the catalystsused in these industries are based on dispersed particles of certainactive components, where the active components are commonly metals orcombinations of metals and other elements. In catalysts of this type,the catalytic properties of the materials are determined by both thetype of active components selected, i.e., the elemental composition ofthe catalyst, and the detailed structure of the dispersed particles,i.e., the atomic scale structure and orientation of the dispersedparticles.

Historically, much of the work in the development and optimization ofcatalysts has focused on the selection of the appropriate catalyticcomponents. Prior methods have allowed catalyst developers to controlthe selection and relative amounts of catalyst components. However, thecontrol of the detailed structure of catalysts, particularly on theatomic scale, has presented a much greater difficulty. Controlling theatomic scale structure can be as important in the development ofeffective catalysts as selecting the elemental composition. For example,control of the detailed catalyst crystal structure can relate directlyto the selectivity of the catalyst. A method which allowed for thecontrolled exposure of certain kinds of catalytic active sites wouldallow certain reaction pathways to be favored to an extent that is notcurrently possible using a catalyst that contains a mixture of differenttypes of active sites.

One particularly useful way of defining a preferred catalytic structureis based on the geometry of the surface active sites. Because ofthermodynamic considerations, it is normally the case that particles ofcrystalline materials will expose one or more of a limited number oflow-index crystal faces. Common low-index crystal face exposures ofmetal particles include, for example, the 111, 100, and 110 crystalfaces of the common crystal lattices, which include face-centered cubic(FCC), body-centered cubic (BCC), and hexagonal close-packed (HCP).Exemplary crystal faces are schematically illustrated in FIG. 1. Each ofthese crystal faces has a different arrangement of atoms, and maytherefore display different catalytic properties with respect to certainchemical reactions. Therefore, substantial improvements in catalystfunction could theoretically be achieved if a method were available toexert effective control over the atomic-scale structure of catalyticparticles. A more detailed description of metal crystal surfacestructure can be found by accessing the National Institute of Standardsand Technology (NIST) WWW home page, particularly the Surface StructureDatabase (SSD).

Despite the extensive history of catalyst development, there are few, ifany, reliable methods which allow the detailed crystal structure ofdispersed catalytic particles to be controlled as a way of improving andoptimizing catalytic function. In part, this derives from the intrinsicdifficulty of controlling structures at an atomic scale. It is alsorelated to a lack of methods to accurately determine whether a desiredatomic scale structure has been successfully achieved. Moreover, asdifficult as it might be to control the shape of the catalyst crystallattice, one of skill in the art would find it even more difficult tocontrol the crystal face exposure of a catalyst crystal.

Attempts have been made to control the crystal lattice structure ofactive catalyst particles. An article by Termer S. Ahmadi, et al. ofGeorgia Institute of Technology, entitled “Shape-Controlled Synthesis ofColloidal Platinum Nanoparticles”, published in Science, Vol. 272, pp.1924–26, describes a method for the synthesis of shape-controlledplatinum particles by controlling the ratio of the concentration ofshaping material to that of ionic platinum. “Tetrahedral, cubic,irregular-prismatic, icoshedral, and cubo-octahedral particle shapeswere observed, whose distribution was dependent on the concentrationratio of the capping polymer material to the platinum cation.” Id, p.1924. The article is silent, however, with respect to how to controlcrystal face exposure of a given crystal shape. Moreover, the articlenot only fails to teach how to select or increase the preponderance ofone crystal face exposure of a catalyst crystal structure over another,it provides no teaching or suggestion that would motivate the selectionof any particular crystal face exposure over another.

BRIEF SUMMARY OF THE INVENTION

The present invention is directed to precursor compositions used to makea range of supported catalysts that have increased specificity forcertain reactions and methods for manufacturing such precursorcompositions. The inventive precursor compositions are designed to yieldsupported catalysts in which a preponderance of the top or outer surfaceof reactive catalyst atoms have a nearest neighbor coordination numberof 2. That is, the catalytically reactive atoms in the supportedcatalyst are arranged so that a preponderance of individual catalystatoms are coordinated with exactly two other catalyst atoms within thetop or outer layer.

Providing catalytically reactive atoms having a coordination number of 2on the reactive surfaces of a supported catalyst greatly limits howchemical reactants in a given chemical reaction are arranged anddistributed on the catalyst surface during the catalyzed chemicalreaction. Limiting how the chemical reactants are arranged anddistributed by the catalyst atoms directly influences and controls therange and type of possible reaction products that can be produced giventhe concentration and identity of chemical reactants. By way of exampleand not limitation, although oxygen and water can react together to formboth water (H₂O) and hydrogen peroxide (H₂O₂), water isthermodynamically favored and will usually be formed over hydrogenperoxide absent some way to alter the reaction conditions. This is alsotrue in the case of catalyzed reactions between oxygen and hydrogen, inwhich the formation of water is naturally favored. It has now beendiscovered that providing a catalyst in which a preponderance of thecatalytically reactive atoms are arranged so as to have a coordinationnumber of 2 on the top or outer layer exhibits high selectivity thatfavors the production of hydrogen peroxide over water using a feedstream comprising oxygen and hydrogen.

In one embodiment, the precursor compositions according to the inventioninclude one or more catalyst atoms complexed with one or more control ortemplating agents to form an intermediate catalyst complex that isselected so as to promote the formation of supported catalysts in whicha preponderance of the catalytically reactive atoms have a coordinationnumber of 2 in the top or outer layer. According to another embodiment,precursor compositions according to the invention may further includeone or more liquid solvents, carriers or dispersing agents. According toyet another embodiment, precursor compositions according to theinvention may further include one or more support particles orsubstrates.

The inventive precursor compositions, together with methods fordepositing catalytically reactive atoms onto a support, control whichface of the catalyst crystallite is predominantly exposed. Examples oflow-index crystal faces having a coordination number of 2 include the(110) crystal face of face centered cubic (FCC) or hexagonal closedpacked (HCP) crystal lattices, which includes linear rows of atoms inthe top or outer layer, the (101), (122), or (120) crystal face of a HCPcrystal lattice, and (112), (122) or (123) crystal face of abody-centered cubic (BCC) crystal lattice. Forming a catalyst crystal orcrystallite may inevitably occur and is the currently understood methodfor producing catalyst particles in which a top or outer layer ofcatalyst atoms have a coordination number of 2. Nevertheless, it is, atleast theoretically, not essential for the catalytically reactive atomsto form a catalyst crystal per se, only that a preponderance of the topor outer layer of atoms in the catalyst particles attached to thesupport have a coordination number of 2. It may be possible to obtainincreased reaction specificity regardless of whether the atoms arealigned as straight rows, in a zig-zag formation, or in less orderedrows having no uniform shape so long as a preponderance of thecatalytically reactive atoms have a coordination number of 2.

According to one embodiment, at least about 50% of the catalyticallyreactive atoms formed using the inventive precursor compositions willadvantageously have a coordination number of 2 in the top or outer layerof atoms. Preferably, at least about 60% of the catalytically reactiveatoms will have a coordination number of 2 in the top or outer layer ofatoms, more preferably at least about 70% of the catalytically reactiveatoms in the top or outer layer, more especially preferably at leastabout 80% of the catalytically reactive atoms in the top or outer layer,and most preferably at least about 90% of the catalytically reactiveatoms in the top or outer layer. Reaction selectivity would be expectedto be even further increased where at least about 95% of thecatalytically reactive atoms in the top or outer layer have acoordination number of 2. Reaction selectivity would be maximized in thecase where 100% of the catalytically reactive atoms in the top or outerlayer have a coordination number of 2

Supported catalysts according to the invention, in addition to providingreactive catalyst atoms in which a preponderance of the atoms in the topor outer layer have a coordination number of 2, may include individualcatalyst particles that (a) are of small size (e.g., as small as 10nanometers or less), (b) have uniform size, shape, and distribution, and(c) are reliably anchored to the support so as to resist agglomerationand/or crystal face reorientation.

A variety of different elements can be used as the catalyst atoms withinthe inventive precursor compositions. For example, the catalyst atomsmay include one or more noble metals, base transition metals, rare earthmetals, and even non-metals. In addition to the foregoing catalystatoms, alkali metals and alkaline earth metals may be present. Theforgoing catalyst atoms can be utilized alone or in combination asdesired to yield a final supported catalyst having a desired catalyticreactivity and/or selectivity relative to one or more targeted chemicalreactions.

The invention contemplates the use of a variety of different control ortemplating agents that, when complexed with one or more catalyst atomsto form an intermediate catalyst complex, can be used in the formationof supported catalysts in which the top or outer layer of catalyticallyreactive atoms have a coordination number of 2. In one aspect, thecontrol or templating agents are capable of forming complexes with thedesired catalyst atoms in a precursor solution. Because of specificstructural and chemical properties, the control agents mediate in theformation of catalyst particles or crystallites, causing thepreferential formation of specific and desirable structures.Specifically, the intermediate catalyst complex interacts duringcatalyst particle formation to induce the formation of dispersedparticle structures with a predominant exposure of controlledcoordination crystal face structures with a top or outer layer ofcatalytically reactive atoms have a nearest neighbor coordination numberof 2.

Control agents within the scope of the invention include a variety ofdifferent polymer, oligomer, or organic molecules. Each control agentmolecule has a structural backbone along which are disposed a pluralityof functional groups for complexing the catalytically reactive atoms tothe control agent. In some cases, the catalyst atoms may be complexed byfunctional groups provided by two or more different compounds orpolymers. For example, 8 catalyst atoms having a valence of 2 might becomplexed on one side by 8 functional groups provided by a firstpolymer, oligomer, or organic molecule and on an opposite side by 8functional groups provided by a second polymer, oligomer, or organicmolecule.

It has now been found that the tendency of the catalytically reactiveatoms to be arranged on a support so as to have a coordination number of2 on the top or outer layer is at least partially determined by thepercentage of straight-chained molecules comprising the control agent,as opposed to molecules that are branched. More specifically, increasingthe percentage of straight-chained molecules has been found to increasethe tendency of the catalytically reactive atoms to have a coordinationnumber of 2. In view of this, the control agent typically includespolymer, oligomer, or organic molecules in which at least about 50% arestraight-chained. In a preferred embodiment, at least about 60% of thepolymer, oligomer, or organic molecules of the control agent will bestraight-chained, more preferably at least about 75%, even morepreferably at least about 90%, and most preferably at least about 95%.The tendency of the catalytically reactive atoms to be arranged so as tohave a coordination number of 2 on the top or outer layer will bemaximized where 100% of the polymer, oligomer, or organic moleculescomprising the the control agent are straight-chained.

In some cases, the tendency of a control polymer, oligomer or organicmolecule to be straight-chained increases with decreasing molecularweight. An example of a control polymer or oligomer that is more linearwith reduced molecular weight is polyacrylic acid. Decreasing themolecular weight of polyacrylic acid decreases its length, which, inturn statistically reduces the likelihood that a particular polyacrylicacid polymer or oligomer will be branched. The use of excess controlagent is believed to result in catalytically active particles that aresmaller in size and more evenly dispersed on the support surface.

Depending on how the final supported catalyst is formed, the controlagent may also act to chemically anchor the reactive catalyst particlesto a support. Preferably, the support has a plurality of hydroxyl orother functional group on the surface thereof that are able tochemically bond to one or more functional groups of the control agent,such as by a condensation reaction. One or more additional functionalgroups of the control agent are also bonded to one or more atoms withinthe catalyst particle, thereby anchoring the catalyst particle to thesupport. Reliably anchoring the catalyst particles to the support helpskeep the catalyst active over time by reducing the tendency of thecatalyst particles to become agglomerated together, which would reducethe available surface area of the catalyst particles. Anchoring thecatalyst particles to the support also reduces or eliminates thetendency of the catalyst particles to become separated entirely from thesupport, which may also reduce the efficacy of the catalyst.

It is within the scope of the invention to arrange catalyst particlesaccording to the invention on any known support. Support substrates maybe 2-dimensional or 3-dimensional, may be porous or nonporous, and maycomprise organic or inorganic materials. The support may itself becatalytic or it may be inert. Exemplary support materials include, butare not limited to, alumina, silica, titania, kieselguhr, diatomaceousearth, bentonite, clay, zirconia, magnesia, as well as the oxides ofvarious other metals, alone or in combination. They also include theclass of porous solids collectively known as zeolites, natural orsynthetic, which have ordered porous structures. Another important classof supports preferred for some applications are carbon-based materials,such as carbon black, activated carbon, graphite, fluoridated carbon,and the like. The support material may also be constructed of a metal ormetal alloy.

The complexed catalyst atoms are generally in the form of a metal saltsolution or colloidal suspension including, but not limited to,chlorides, nitrates, phosphates, sulfates, tungstates, acetates,citrates, and glycolates. A solvent may be used as a vehicle for eitherthe catalytic component or the control agent. Preferred solvents includewater, dilute aqueous acids, methanol, ethanol, normal and isopropanol,acetonitrile, acetone, tetrahydrofuran, ethylene glycol,dimethylformamide, dimethylsulfoxide, methylene chloride, and the like,including mixtures thereof. The precursor solution or colloidalsuspension may be acidified with any suitable acid, including organicand inorganic acids.

Exemplary methods for making precursor compositions according to theinvention include providing catalyst atoms in solution (e.g., in theform of an ionic salt), providing a control agent in solution (e.g., inthe form of a carboxylic acid salt), and reacting the catalyst atomswith the control agent to form a precursor composition comprising anintermediate catalyst complex of the catalyst atoms and the controlagent. In one aspect of the invention, the “precursor composition” maybe considered to be the intermediate catalyst complex formed from thecatalyst atoms and the control agent, exclusive of the surroundingsolvent or carrier. Indeed, it is within the scope of the invention tocreate such a complex in solution, or as a colloidal suspension, andthen remove the solvent or carrier so as to yield a solid precursorcatalyst complex that can be later added to an appropriate solvent orcarrier to reconstitute a solution or colloidal suspension containingthe catalyst complex. Thus, in another aspect of the invention,“precursor compositions” according to the invention may include one ormore different solvents or carriers into which an intermediate catalystcomplex has been dispersed.

In order to form a supported catalyst, the intermediate catalyst complexis applied to a support, typically by means of an appropriate solvent orcarrier in order to apply or impregnate the catalyst complex onto thesupport. Thus, in yet another aspect of the invention, “precursorcompositions” according to the invention may include the catalystcomplex, a solvent or carrier, and a support onto which the catalystcomplex is applied. Thereafter, the solvent or carrier is removed,optionally in connection with a reaction step that causes the controlagent to become bonded to the support. This yields a supported precursorcatalyst in which the catalyst atoms have been arranged in a desiredfashion, but not yet exposed or otherwise activated.

In order to expose at least a portion of catalyst atoms and yield anactive supported catalyst, a portion of the control agent is removed,such as by reduction, e.g., hydrogenation. The resulting catalyst can beoptionally heat-treated to further activate the catalyst atoms. In apreferred embodiment, the process of removing the control agent toexpose the catalyst atoms is carefully controlled to ensure that enoughof the control agent remains so as to reliably anchor the catalystparticles to the support. Thus, at least that portion of the controlagent interposed between the support and the bottom surface of thecatalyst particles facing the support is advantageously left intact.What remains of the control agent may be considered to comprise an“anchoring agent.” (In addition to the remaining portion of the controlagent, the “anchoring agent” may optionally comprise other polymers,oligomers or organic compounds that are interposed between the catalystparticles and support and that aid in anchoring the catalyst particlesto the support.)

On the other hand, removing the control agent to the extent that littleor any of it remains to anchor the catalyst particles to the support hasbeen found to reduce the long-term stability of the supported catalyst.Removing all of the control agent may greatly reduce the long-termstability of the supported catalyst. Whereas removing all of the controlagent may initially yield catalyst particles having the desired crystalface exposure, leaving behind insufficient control agent and/or failingto provide another polymer, oligomer, or organic compound to reliablybind the catalyst particles to the support results in catalyst particlesthat are considerably more mobile. This, in turn, results in a greatertendency of the catalyst particles to become agglomerated when exposedto heat (e.g., such as when the catalyst is in use) and/or separatedfrom the support altogether.

By way of example, supported catalysts according to the invention may beuseful in catalytically promoting the following chemical reactions:

-   -   1. Reactions involving hydrogen and oxygen to selectively form        hydrogen peroxide instead of water. The hydrogen peroxide so        formed may be used or sold as a product or used as an oxidizing        agent in an integrated manufacturing process to form other        chemical products;    -   2. Reactions involving hydrogen, oxygen, and organic compounds        to form chemical products, e.g., in situ formation of hydrogen        peroxide as an intermediate oxidizing agent that reacts with the        organic compound to form other chemical products;    -   3. Reactions involving oxygen and organic compounds to form        oxidized chemical products, i.e., direct oxidation without        hydrogen peroxide as an intermediate;    -   4. Reactions between hydrogen and organic compounds to form        chemical or fuel products, i.e., hydrogenation, hydrotreating        and hydrocracking;    -   5. Reactions of chemical products to liberate hydrogen, i.e.,        dehydrogenation and reforming; and    -   6. Electrochemical reactions of hydrogen and/or oxygen at fuel        cell electrodes.

These and other advantages and features of the present invention willbecome more fully apparent from the following description and appendedclaims, or may be learned by the practice of the invention as set forthhereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

To further clarify the above and other advantages and features of thepresent invention, a more particular description of the invention willbe rendered by reference to specific embodiments thereof which areillustrated in the appended drawings. It is appreciated that thesedrawings depict only typical embodiments of the invention and aretherefore not to be considered limiting of its scope. The invention willbe described and explained with additional specificity and detailthrough the use of the accompanying drawings, in which:

FIGS. 1A and 1B schematically depict exemplary low-index crystal faces;

FIGS. 2A and 2B schematically compare how hydrogen and oxygen arearranged or coordinated relative to catalyst atoms on differentlow-index crystal faces;

FIG. 3 is a schematic diagram of a top view of an exemplary top or outerlayer of reactive catalyst atoms having a coordination number of 2 andarranged in non-regularly arranged rows;

FIG. 4 is a schematic diagram of the chemical structure of an exemplaryintermediate catalyst complex of the present invention;

FIG. 5 schematically illustrates a catalyst particle chemically bondedto a support by an anchoring agent;

FIG. 6 is a high resolution TEM of a supported Pd—Pt/C catalystaccording to the invention showing the ordered arrangement of the noblemetal catalyst atoms; and

FIG. 7 is a high resolution TEM of a supported Pd—Pt/TS1 catalystaccording to the invention showing the ordered arrangement of the noblemetal catalyst atoms on a TS1 catalyst support.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

I. Introduction and Definitions

The present invention is directed to intermediate precursor compositionsfor making supported catalysts in which a preponderance of the top orouter layer of reactive catalyst atoms have a nearest neighborcoordination number of 2. Supported catalysts made using the inventiveprecursor compositions exhibit high specificity for a variety ofdifferent chemical reactions. The invention also relates to methods formanufacturing such intermediate precursor compositions.

The term “precursor composition” refers to a range of intermediatecompositions that are used to manufacture reactive supported catalystshaving a controlled coordination structure. The precursor compositionsthemselves may not be catalytically active unless further processed.Examples of inventive precursor compositions include (1) intermediatecatalyst complexes comprising catalyst atoms complexed with a controlagent; (2) solutions or colloidal suspensions comprising an intermediatecatalyst complex and a solvent or carrier; (3) solutions or suspensionscomprising an intermediate catalyst complex, solvent or carrier, and asupport (whether or not the catalyst complex is actually bonded to thesupport); and (4) an intermediate catalyst complex bonded to a supportin the absence of a solvent or carrier.

The terms “intermediate catalyst complex” and “catalyst complex” referto a solution, colloid or suspension in which a bond or coordinationcomplex is formed between a control or templating agent and one or moredifferent types of catalyst atoms. The “bond” between the control agentand catalyst atoms may be ionic, covalent, electrostatic, or it mayinvolve other bonding forces such as coordination with nonbondingelectrons, Van Der Waals forces, and the like.

The terms “control agent” and “templating agent” refer to a class ofpolymers, oligomers, or organic compounds which promote the formation ofsupported catalyst particles having the controlled coordinationstructure of the present invention. Within the intermediate precursorcomposition, the control or templating agent first forms an intermediatecatalyst complex with the catalyst atoms in order to form a solution ordispersion of catalyst atoms in a solution or colloidal suspension. Whendeposited onto a support, the intermediate catalyst complex forms anordered array of catalyst atoms. Upon removal of a portion of thecontrol agent, a preponderance of the exposed catalyst atoms arearranged so as to have a nearest neighbor coordination number of 2.

The term “catalyst atom” refers to metallic or non-metallic atoms thatare complexed with the control or templating agent to form anintermediate catalyst complex. After removing a portion of the controlagent, the catalyst atoms become “reactive catalyst atoms” that are ableto catalytically promote a desired chemical reaction.

The term “catalyst particle” refers to the catalytically active portionof a supported catalyst. In some (or perhaps) all cases, the catalystparticles will be “crystal particles” in which the reactive catalystatoms are arranged in an ordered crystal structure. Nevertheless, it iswithin the scope of the invention to form supported reactive catalystsin which the active catalyst particles are not arranged in an orderedcrystal structure. A distinguishing feature of the inventive catalystparticles according to the invention is that a preponderance of reactivecatalyst atoms on a top or outer layer of the catalyst particles willhave a nearest neighbor coordination number of 2.

The term “crystal face” refers to the top or outer layer of reactivecatalyst atoms within a catalyst crystal. The terms “crystal faceexposure” and “crystal face exposition” refer to the specificarrangement of catalyst atoms within a particular crystal face (e.g, lowindex crystal face exposures (100), (110), and (111)).

The terms “controlled phase exposition” or “controlled face exposure”are used herein to refer to the situation where a catalytic crystal orparticle has a top or outer layer of catalyst atoms in desiredcoordination structure.

The term “coordination number of 2” refers to a crystal face exposure,whether low or high index, in which the reactive catalyst atoms on thetop or outer layer are arranged so that each atom, except the terminalatoms in a given row, is coordinated with exactly 2 other catalystatoms. The terminal atoms of each row will, of course, be coordinatedwith only 1 other catalyst atom, yet are considered to have a“coordination number of 2” for purposes of determining the percentage oftop or outer layer catalyst atoms that have a nearest neighborcoordination number of 2. In the case of catalyst particles that are notreally crystals, the reactive catalyst atoms in top or outer layer mayor may not actually emulate a true crystal face.

The term “supported reactive catalyst” refers to a compositioncontaining one or more catalyst particles attached to a support. Thesupport itself may be catalytically active or it may be catalyticallyinert.

II. Overview of Supported Reactive Catalysts

Supported reactive catalysts that can be manufactured using theinventive precursor composition include catalyst particles in which apreponderance of catalyst atoms in the top or outer layer have a nearestneighbor coordination number of 2. To better understand what is meant bythe term “coordination number of 2” and why reactive catalyst atoms soarranged have high selectivity for certain reactions, reference is madeto FIGS. 1 and 2.

FIGS. 1A and 1B schematically illustrate crystal face exposures that areeither desired or undesired. The desired presentation is distinguishedby exhibiting predominantly a coordination number of 2 for the top orouter layer of catalyst atoms of the crystal face, in contrast to theundesired crystal face exposure having a higher coordination number. TheFCC (110) face and HCP (120) face exposures of FIG. 1A are examples ofdesired atomic arrangements in which the top or outer layer atoms have acoordination number of 2. The top or outer layer of atoms in the FCC(110) face exposure are arranged in a linear fashion, while the top orouter layer of atoms in the HCP (120) face exposure are arranged in azig-zag fashion. In each case, each catalyst atom within the top orouter layer is coordinated with exactly 2 other catalyst atoms (exceptfor the terminal atoms in each row, which are coordinated with 1catalyst atom). As discussed more fully below, this arrangement ofcatalyst atoms favors the formation of hydrogen peroxide instead ofwater when the catalyst is used to catalyze a reaction using a feedstream containing oxygen and hydrogen.

In contrast to the favored face exposures, the FCC (100) face and FCC(111) face exposures of FIG. 1B are examples of less desired orundesired arrangements because the top or outer layer of catalyst atomsare coordinated with more than 2 other catalyst atoms. In the case ofthe FCC (100) face exposure, each interior catalyst atom in the top orouter layer is coordinated with 4 other catalyst atoms. For the FCC(111) face exposure, each interior catalyst atom in the top or outerlayer is coordinated with 6 other catalyst atoms. Both FCC (100) and(111) face exposures are non-selective for hydrogen peroxide over water.Since water is thermodynamically favored (i.e., because it is much morestable than hydrogen peroxide), both FCC (100) and (111) face exposuresfavor the formation of water.

To better illustrate how catalyst particles in which the top or outlayer of reactive catalyst atoms have a nearest neighbor coordinationnumber of 2 favor the formation of the less thermodynamically favoredhydrogen peroxide over water, reference is made to FIGS. 2A and 2B. Asseen in FIG. 2A, catalyst atoms having the desired exposure allow forcontrolled surface adsorption of hydrogen and oxygen on the catalystparticle surface in a presentation which, at least in theory, onlyallows for the production of hydrogen peroxide. Both hydrogen and oxygenexist as diatomic molecules that contain two atoms of hydrogen (H—H, orH₂) or two atoms of oxygen (O═O, or O₂), respectively, (where “—”denotes a single bond and “═” denotes a double bond). Because oxygennormally forms two bonds, but hydrogen only forms one bond, whenadsorbed onto or coordinated with catalyst atoms in FCC (110) and HCP(120) face exposures, the only arrangement of hydrogen and oxygenmolecules that promotes a reaction between individual hydrogen andoxygen atoms is (H O O H), as shown in FIG. 2A. Upon formation ofmolecular bonds between the coordinated hydrogen and oxygen atoms, ahydrogen peroxide molecule (H—O—O—H) is formed.

The only other possible arrangements of hydrogen and molecules relativeto every group of four catalyst atoms having a coordination number of 2,i.e., (H H H H), (O O O O) and (H H O O), do not promote reactionsbetween hydrogen and oxygen. No arrangements of H₂ and O₂ moleculespromote the formation of water because that would require a lone oxygenatom to be coordinated with two hydrogens (H O H) to form water (H—O—H).However, lone oxygen atoms are not normally found within a feed streamof oxygen gas under typical reaction conditions. Thus, hydrogen peroxideformation is favored over water.

In contrast to FCC (110) and HCP (120) face exposures, an undesiredsurface coordination of the top-layer atoms, e.g., FCC (100) and FCC(111) face exposures, does not exhibit specificity to the production ofhydrogen peroxide over water. FIG. 2B depicts possible arrangements ofoxygen and hydrogen atoms when adsorbed onto or coordinated withcatalyst atoms in both the FCC (110) and HCP (120) face exposures. Whenoxygen and hydrogen are so arranged, only water is formed. Admittedly,FIG. 2B depicts the worst-case scenario for the production of hydrogenperoxide. Given a random distribution of hydrogen and oxygen moleculesadsorbed onto or coordinated with catalyst atoms in FCC (110) and HCP(120) face exposures, both hydrogen peroxide and water can be produced.However, because water is far more stable, its production isthermodynamically favored over hydrogen peroxide.

In order to overcome the tendency of conventional catalysts to producewater instead of hydrogen peroxide, other strategies must be employed toobtain a commercially viable concentration of hydrogen peroxideformation. For example, a feed stream substantially richer in oxygenthan in hydrogen can increase the likelihood that the oxygen andhydrogen atoms can be arranged on the catalyst surface so as to formhydrogen peroxide. However, substantially altering the relativeconcentrations of oxygen and hydrogen in this manner can slow down thereaction and decrease yields. In contrast, employing a catalyst in whicha preponderance of catalyst atoms have a coordination number of 2 favorsthe production of hydrogen peroxide over water by virtue of the catalystalone. This is a huge advantage over conventional catalysts.

It should be understood that the crystal phase exposures illustrated inFIGS. 1 and 2 are non-limiting examples of catalyst atoms in the top orouter layer having a coordination number of 2. The present inventioncontemplates any arrangement of top or outer layer atoms exhibiting anearest neighbor coordination number of 2, i.e., in which all othernearby atoms are spaced at greater than nearest neighbor spacing, arenot located in the top or outer layer of atoms, or both. For thepurposes of this invention, the nearest neighbor spacing referred to inthe above description is the actual nearest neighbor spacing within thetop atomic layer of the crystal. This spacing may be similar to thenearest neighbor spacing of the bulk crystal lattice of the catalyticcomponent, but may differ from that bulk spacing to some extent becauseof crystal structure deformations or relaxations that may occur atexposed surfaces of crystalline materials.

The preferred coordination structure is believed to encourage reactionsbetween adsorbed components located on nearest neighbor coordinatedsites, while discouraging or substantially preventing reactionsoccurring between non-coordinated sites located where there is greaterthan nearest neighbor spacing. By restricting the number of nearestneighbor sites available for intermolecular or interatomic reactions onthe catalyst surface, the controlled coordination structure is believedto be responsible for the very high selectivity which can be achievedusing catalysts of this invention. For instance, a selectivity of up to100% can be achieved in one application of the invention wherein acatalyst comprising palladium including top or outer surface atomshaving a coordination number of 2 is employed to produce hydrogenperoxide from hydrogen and oxygen gas feed streams.

As discussed above, controlled coordination structures according to theinvention may include linear configurations, such as the FCC (110) faceshown in FIG. 1A, or a zigzagged configuration, such as the HCP (120)face also shown in FIG. 1A. The controlled coordination structures ofthe present invention also includes several families of low-indexcrystal faces which have been found to have suitable coordinationstructures and are useful structures for the catalysts of thisinvention. Examples include:

(a) the (110) face of the FCC (face-centered cubic) lattice,

(b) the (221), (331) and (332) crystal faces of the FCC lattice;

(c) the (110) crystal face of the HCP (hexagonal closed packed) lattice,including (220), (330), etc.

(d) the (101) crystal face of the HCP lattice, including (202), (303),etc.

(e) the (122) crystal face of the HCP lattice;

(f) the (120) crystal face of the HCP lattice;

(g) the (122) crystal face of the BCC (body-centered cubic) lattice; and

(h) the (112) and (123) crystal face of the BCC lattice.

In all of the above crystal face designations, it will be understood bythose skilled in the art that each named crystal face has many alternateMiller index designations, each of which are equivalent to those listedabove. All of the unnamed but equivalent crystal face designationsshould be understood to be included within the scope of this invention.For example, in the FCC and BCC crystal lattices, all three coordinatedirections are equivalent. In this example, the (110) crystal face isidentical to the (101) and the (011) crystal faces. For the HCP lattice,only the first two coordinates are equivalent. For example, the (101)and the (011) crystal faces are identical, while the (110) crystal faceis distinct.

Theoretically, the term “coordination number of 2” may includecatalytically reactive atoms in any arrangement so long as the catalystatoms in the top or outer layer have a nearest neighbor coordinationwith only 2 catalyst atoms. As shown in FIG. 3, alternative arrangementsof top or outer layer atoms exhibiting a nearest neighbor coordinationnumber of 2 may include less ordered rows that are not normallyassociated with an ordered crystal lattice. So long as the top or outerlayer of catalyst atoms have a nearest neighbor coordination number of2, catalyst particles of that type should still favor the production ofhydrogen peroxide over water.

According to one embodiment, at least about 50% of the reactive catalystatoms in the top or outer layer in a supported reactive catalyst willadvantageously have a coordination number of 2. Preferably, at leastabout 60% of the reactive catalyst atoms in the top or outer layer willhave a coordination number of 2, more preferably at least about 70% ofthe reactive catalyst atoms in the top or outer layer, more especiallypreferably at least about 80% of the reactive catalyst atoms in the topor outer layer, and most preferably at least about 90% of the reactivecatalyst atoms in the top or outer layer will have a coordination numberof 2. Reaction selectivity would be expected to be even furtherincreased where at least about 95% of the reactive catalyst atoms in thetop or outer layer have a coordination number of 2. Reaction selectivitywould be maximized in the case where 100% of the reactive catalyst atomsin the top or outer layer have a coordination number of 2.

The catalyst particles of the present invention may be present in anysize, but it is preferred that they be of small size, in the 1–100 nmsize range. While not essential, it is also preferred that the catalyticparticles be of generally uniform size. Furthermore, it is alsopreferred that the catalytic particles be uniformly dispersed on thesupport to reduce the effects of interparticle interaction, which canlead to undesirable effects, such as loss of activity or loss ofselectivity due to, e.g., particle agglomeration or sintering.

In the case where two or more different types of catalyst atoms areused, it is possible, according to the concepts described herein, forsupported catalysts to include an even distribution of catalyst atoms.This is a significant departure from conventional methods ofmanufacturing supported catalysts, in which different catalyst atoms aretypically clustered together due to the natural tendency of catalystatoms to attract and congregate with like atoms. Thus, in a conventionalcatalyst comprising a mixture of palladium and platinum, the majority ofpalladium atoms will be clustered together in crystals where palladiumpredominates, and the the majority of platinum atoms will be clusteredtogether in crystals where platinum predominates. In contrast, theability to complex individual atoms using the control or templatingagent, as discussed more fully below, allows for intimate, even a highlyrandomized, mixing of two or more different types of catalyst atoms.

The supported reactive catalysts having a controlled coordinationstructure are useful in various reactions, including formation ofhydrogen peroxide, formation and use of hydrogen peroxide as anintermediate reactant in the oxidation of organic molecules, directoxidation of organic molecules, hydrogenation, reduction,dehydrogenation, and, potentially, to make fuel cells.

The catalytic materials, control agents, support materials, and othercomponents which may be selected to form catalytic particles accordingto the present invention will now be discussed in detail.

III. Precursor Compositions

The inventive precursor compositions may include any component that aidsin the formation of supported catalysts in which the catalyst particlesinclude a preponderance of catalyst atoms in the top or outer layerhaving the desired controlled coordination structure. Examples include,but are not limited to, catalyst atoms, control or templating agents,intermediate catalyst complexes formed from catalyst atoms and controlagents, solvents or carriers, and support materials.

A. Intermediate Catalyst Complexes

Intermediate catalyst complexes include one or more different types ofcatalyst atoms complexed with one or more different types of control ortemplating agents. When so complexed, the catalyst atoms are arranged insuch a manner that, when the intermediate catalyst complex is used tomanufacture a supported reactive catalyst and a portion of the controlagent is removed to expose a portion of the catalyst atoms in a desiredmanner, that a preponderance of reactive catalyst atoms will have adesired controlled coordination structure.

1. Catalyst Atoms

Any element or group of elements that can exhibit catalytic activity canbe used to form catalyst complexes and catalyst according to theinvention. These include elements or groups of elements that exhibitprimary catalytic activity, as well as promoters and modifiers. As theprimary catalytic active component, metals are preferred. Exemplarymetals can include noble metals, base transition metals, and rare earthmetals. Catalyst particles may also comprise non-metal atoms, alkalimetals and alkaline earth metals, typically as modifiers or promoters.

Examples of base transition metals that may exhibit catalytic activityinclude, but are not limited to, chromium, manganese, iron, cobalt,nickel, copper, zirconium, tin, zinc, tungsten, titanium, molybdenum,vanadium, and the like. These may be used alone, in various combinationswith each other, or in combinations with other elements, such as noblemetals, alkali metals, alkaline earth metals, rare earth metals, ornon-metals.

Examples of noble metals, also referred to as platinum-group metals,that exhibit catalytic activity, include platinum, palladium, iridium,gold, osmium, ruthenium, rhodium, rhenium, and the like. These may beused alone, in various combinations with each other, or in combinationswith other elements, such as base transition metals, alkali metals,alkaline earth metals, rare earth metals, or non-metals.

Noble metals are particularly well-suited for use in this invention fora variety of reasons. They are highly active catalysts which are usefulin numerous chemical transformations. When dispersed, they can formsmall crystallites on which the desired controlled coordination surfacestructure can be obtained having one or more of the known low-indexcrystal faces of the noble metal crystal lattice. Because they are veryexpensive, it is particularly desirable to make the most efficientpossible use of the noble metal constituents by assuring that apreponderance of the most desired active sites are exposed.

Examples of rare earth metals that exhibit catalytic activity include,but are not limited to, lanthanum and cerium. These may be used alone,in various combinations with each other, or in combinations with otherelements, such as base transition metals, noble metals, alkali metals,alkaline earth metals, or non-metals.

Examples of non-metals include, but are not limited to, phosphorus,oxygen, sulfur and halides, such as cholorine, bromine and fluorine.These are typically included as functionalizing agents for one or moremetals, such as those listed above.

The preferred catalytically active component will depend on the specificapplication. For example, one advantageous use of the catalyst of thisinvention is the direct synthesis of hydrogen peroxide from hydrogen andoxygen, either as a product or as a chemical intermediate in thesynthesis of other chemical products. In this application, the preferredcatalyst active component is palladium, either alone or in combinationwith other metals (e.g., platinum).

Another useful application of the invention is catalytic hydrogenation,in which a preferred primary active component is platinum, palladium,nickel, cobalt, copper or iron, either alone or in combination with eachother and/or other components.

Another useful application of the invention is catalytic reforming, inwhich the preferred catalytic active component is platinum, which may beused alone or in combination with other components. Platinum is also apreferred primary active component when the subject catalyst is used asthe cathode and/or anode catalyst in a fuel cell, such as a polymerelectrolyte membrane (PEM) or a direct methanol fuel cell (DMFC).

When added to an appropriate solvent or carrier to form an intermediateprecursor composition, the catalyst atoms will typically be in ionicform so as to more readily dissolve or disperse within the solvent orcarrier. In the case of a metallic catalyst, the catalyst atoms may bein the form of a metal halide, nitrate or other appropriate salt that isreadily soluble in the solvent or carrier, e.g., metal phosphates,sulfates, tungstates, acetates, citrates, or glycolates.

2. Control Agents

In order to achieve the highly controlled specificity of catalystformation, a control agent or templating agent is selected to promotethe formation of catalyst crystals or particles in which a preponderanceof the top or outer layer atoms have a coordination number of 2. Throughuse of one or more specific control agents, the present inventionprovides a means to control the process whereby the catalytic particlesare formed, ensuring that these particles predominantly expose a desiredcrystal face.

Control or templating agents within the scope of the invention include avariety of polymer, oligomer or organic compounds, comprising individualmolecules, that mediate in the formation of the dispersed catalystparticles. The control agent molecules include a plurality of functionalgroups disposed along a backbone that are able to form a complex betweenthe catalyst atoms and the control agent. When catalytic particles areformed from the intermediate catalyst complex, the structure,conformation, or other properties of the control or templating agentcause formation of the catalyst particles to proceed in a controlledfashion, favoring the formation of controlled coordination structures.

In general, useful control agents include polymers, oligomers, andorganic compounds that can form catalyst complexes within anintermediate precursor composition that includes the control agent,catalyst atoms, an appropriate solvent or carrier, and optionalpromoters or support materials. The control agent is able to interactand complex with catalyst atoms dissolved or dispersed within anappropriate solvent or carrier through various mechanisms, includingionic bonding, covalent bonding, van der Waals interaction, or hydrogenbonding. In order to do this, the control agent includes one or moreappropriate functional groups.

In one embodiment, the functional groups comprise a carbon atom bondedto at least one electron-rich atom that is more electronegative than thecarbon atom and that is able to donate one or more electrons so as toform a bond or attraction with a catalyst atom. Preferred control agentsinclude functional groups which have either a negative charge or one ormore lone pairs of electrons that can be used to complex a catalystatom. This allows the control agent to have a strong binding interactionwith dissolved catalyst atoms, which are preferably in the form ofpositively charged ions in solution.

In one embodiment, the functional groups of the control agent comprisecarboxyl groups along the backbone of the control agent molecules,either alone or in combination with other types of functional groups. Inother embodiments, the functional groups may include one or more ofhydroxyl, ester, ketone, aldehyde, amine, or amide groups, andcombinations thereof.

Control agents according to the invention are advantageously organicpolymers, oligomers or compounds; however, the control agent may be aninorganic compound (e.g., silicon-based). The control agent may be anatural or synthetic compound. In the case where the catalyst atoms aremetals and the control agent is an organic compound, the catalystcomplex so formed is an organometallic complex.

Preferred control agents include a variety of oligomers and polymers. Inthe case where the control agent is an oligomer or polymer, themolecular weight, measured in number average, is preferably in a rangefrom about 300 to about 15,000 Daltons, more preferably in a range ofabout 600 to about 6000 Daltons. However, it is recognized that evenhigh molecular weight polymers, i.e., greater than 15,000, can be usedas the control agent if they are readily soluble in solvents, carriersor vehicles compatible with the catalyst atoms and able to form anorganometallic complex.

The molecular weight may be selected to yield a control agent polymer,oligomer or molecule having a desired number of functional groups. Ingeneral, the number of functional groups may range from 4 to 200,preferably from about 8 to about 80 functional groups, and morepreferably from about 10 to about 20 functional groups. In many cases,the number of functional groups within a polymer or oligomer at leastapproximately corresponds to the number of repeating units. As statedelsewhere, it may be possible to reduce or minimize branching byselecting a control polymer or oligomer having fewer repeating units,e.g, fewer than 20.

Suitable polymers and oligomers within the scope of the inventioninclude, but are not limited to, polyacrylates, polyvinylbenzoates,polyvinyl sulfate, polyvinyl sulfonates including sulfonated styrene,polybisphenol carbonates, polybenzimidizoles, polypyridine, sulfonatedpolyethylene terephthalate. Other suitable polymers include polyvinylalcohol, polyethylene glycol, polypropylene glycol, and the like.

In the intermediate catalyst complex, the catalyst atoms, more preciselyions of the catalyst atoms, are in an arranged formation correspondingto the functional groups located on the polymer, oligomer or molecularbackbones. For example, as illustrated in FIG. 4, the catalyst atoms maybe complexed on one side to a first polymer or oligomer chain, moreparticularly by the functional groups along the length of the controlagent molecule, and on a second side by a second polymer chain. In thisway the catalyst atoms can be considered to be disposed in a controlledarrangement corresponding to the functional groups along the backbonesof the first and second control agent molecules while in solution or acolloidal suspension even before they are deposited or bonded to asupport.

It may be advantageous to provide an excess of the control agent so asto provide an excess of functional groups relative to the number ofcatalyst atoms. Including an excess of functional groups helps ensurethat all or substantially all of the catalyst atoms are complexed by thecontrol agent, which is particularly beneficial in case where thecatalyst material is expensive, such as in the case of noble metals.Providing an excess of control agent also helps ensure the availabilityof functional groups for bonding the catalyst complex to the support. Itis also believed that employing an excess of functional groups helpsyield a supported catalyst in which the catalyst particles are moreevenly dispersed. Excess control agent molecules are believed tointervene and maintain spacing between control agent molecules that formthe catalyst complex in order to better distribute the individualcatalyst complex clusters over the support surface rather than allowingthem to clump or agglomerate together.

It has now been found that the tendency to form catalyst particleshaving a coordination number of 2 on the surface is related to thepercentage of straight-chained control agent molecules relative tobranched molecules. Increasing the concentration of straight-chainedmolecules increases the likelihood of forming catalyst crystals orparticles that have a coordination number of 2 on the top or outer layerof catalyst atoms. This, in turn, increases the specificity of thedesired catalytic reaction.

The term “straight-chained” denotes a polymer, oligomer, or organiccompound that includes a backbone that does not have any branch points.Thus, it is in reference to the backbone that determines whether aparticular control agent molecule is straight-chained or not. If thebackbone of a control agent molecule is linear without any branchpoints, the molecule is “straight-chained”. If the backbone includesbranch points such that the backbone is not linear but is branched, themolecule is “branched”.

The term “backbone” denotes the portion of the control agent molecule towhich functional groups that are useful in complexing catalyst atoms areattached. Backbones that include stray groups or chains to which nocomplexing functional groups are attached may therefore still beconsidered to be straight-chained. Thus, it may be more precise todetermine whether a particular polymer, oligomer or organic compound isstraight-chained or branched by determining the arrangement offunctional groups along the backbone rather than merely determiningwhether the backbone includes branch points.

In view of the foregoing, typical control agents according to theinvention are those in which at least about 50% of the control agentmolecules are straight-chained. Preferably, at least about 60% of thecontrol agent molecules are straight-chained, more preferably at leastabout 75% of the control agent molecules are straight-chained, even morepreferably at least about 90% of the control agent molecules arestraight-chained, and most preferably, at least about 95% of the controlagent molecules are straight-chained. The tendency of the control agentto yield catalyst particles in which the top or outer layer of catalystatoms have a coordination number of 2 is maximized where about 100% ofthe control agent molecules are straight-chained.

As a corollary to the foregoing, the control agent generally includesless than about 50% branched molecules, preferably less than about 40%branched molecules, more preferably less than about 25% branchedmolecules, even more preferably less than about 10% branched molecules,and most preferably less than about 5% branched molecules. The tendencyof the control agent to yield catalyst particles in which the top orouter layer of catalyst atoms have a coordination number of 2 ismaximized where about 0% of the molecules are branched.

In some cases, the tendency of a polymer, oligomer or organic moleculeto be branched decreases with decreasing molecular weight, morespecifically, with a decreased number of repeating units. Thus, reducingthe molecular weight, or more precisedly the number of repeating units,of a polymer, oligomer or organic molecule increases its tendency to bestraight-chained. An example of a control polymer or oligomer that ismore straight-chained with reduced molecular weight (i.e., fewerrepeating units) is polyacrylic acid. Decreasing the molecular weight ofpolyacrylic acid decreases the number of repeating units, which, in turnstatistically reduces the likelihood that a particular polyacrylic acidpolymer or oligomer molecule will be branched.

For example, polyacrylic acid having a molecular weight of 1200, whichhas approximately 16 repeating units and yields catalyst particleshaving surface diameter of about 3–5 nm, is believed to have minimalbranching. Based on current data, it is believed that at least about80–90% of the molecules comprising polyacrylic acid having a molecularweight of 1200 are straight-chained. This is consistent with teachingsrelating to polymer branching found within, Hiemenz, Polymer Chemistry:The Basic Concepts (1984), p. 394, which states that, for highconversions of polyethylene, “side chains may occur as often as onceevery 15 backbone repeat units on the average.” Thus, at least withrespect to polyethylene and similar polymers, oligomers having fewerthan 15 units might be expected to be entirely straight-chained with nobranch points. Hence, polyacrylic acid oligomers of 16 units would beexpected to have only small incidence of branching, if any, particularlyif reaction conditions are more carefully controlled to reduce theincidence of branching.

Once it is understood that increasing the concentration ofstraight-chained control agent molecules increases the likelihood offorming a catalyst particle in which the top or outer layer of catalystatoms have a coordination number of 2, one of skill in the art will beable to select an appropriate control agent having an appropriateconcentration of straight-chained versus branched control agentmolecules. Thus, even larger molecular weight polymers may be carefullyengineered to have straight chains and thus may have usefulness in thecompositions and methods of the present invention. Most of thestructural aspects of the catalytic particles such as their size, shape,formation, and dispersion can be designed based on selecting one or morecontrol agents or templating agents having a particular size and/orpercentage of straight molecules versus branched molecules.

In addition to the characteristics of the control agent, a second factorwhich can control the selective formation of the desired controlledcoordination structures of the invention is the molar ratio of thecontrol agent to the catalyst atoms in the intermediate precursorcomposition. It is within the scope of the invention to include a molarratio of control agent molecules to catalyst atoms in a range of about1:0.1 to about 1:10. Preferably, the molar ratio of control agentmolecules to catalyst atoms is in a range of about 1:0.2 to about 1:5.

In some cases, a more useful measurement is the molar ratio betweencontrol agent functional groups and catalyst atoms. For example, in thecase of a divalent catalyst metal ion, such as Pd⁺², two molarequivalents of a monovalent functional group, such as carboxylate ion,would be necessary to provide the theoretical stoichiometric ratio. Itmay be desirable to provide an excess of control agent functional groupsto (1) ensure that all or substantially all of the catalyst atoms arecomplexed, (2) bond the catalyst complex to the support, and (3) helpkeep the catalyst particles segregated so that they do not clump oragglomerate together. In general, it will be preferable to include amolar ratio of control agent functional groups to catalyst atoms in arange of about 0.5:1 to about 40:1, more preferably in a range of about1:1 to about 35:1, and most preferably in a range of about 3:1 to about30:1.

It is believed that the foregoing ratios play a factor because thenumber of control agent molecules that surround each catalyst atomdetermines the rate and orientation in which the catalyst particles areformed. It should be noted that the most preferred ratio of controlagent to catalyst atoms will depend on the type of control agent used,the type of catalyst atoms used, and the molecular weight of the controlagent. For control agents with higher molecular weights, a lower ratioof control agent to catalytic component is generally preferred, comparedto cases where control agents have lower molecular weights. It isbelieved that this derives from the fact that control agents will oftenhave multiple points of interaction and complex formation with dissolvedcatalyst components. Therefore, a higher molecular weight control agentwith more functional groups along the backbone of the control agentmolecules can complex with more catalyst atoms, and a lower molar ratioof control agent to catalyst component is preferred. The converse istrue for a control agent with a lower molecular weight, where a highermolar ratio of control agent to catalyst component will therefore bepreferred.

When a support material is added to an intermediate precursorcomposition, the control agent acts to uniformly disperse the complexedcatalyst atoms onto the support material. Because the catalyst atoms aredispersed, the particles resulting from the coalescing of the catalystatoms are also uniformly dispersed because the particles will form inthe most thermodynamically stable formations. This results in a moreactive catalyst since uniformly dispersing the catalytic particlesallows more reactive sites to be exposed.

Finally, depending on how the supported catalyst is formed, anotheraspect of the control or templating agent is that it may act to anchorthe catalyst particles to the support. That is, during and afterformation of the catalyst particles, the control agent may act as ananchoring agent to secure the particle to the substrate material.Preferably, the substrate has a plurality of hydroxyl or otherfunctional groups on the surface thereof which are able to chemicallybond to one or more functional groups of the control agent, such as by acondensation reaction. One or more additional functional groups of thecontrol agent are also bonded to one or more atoms within the catalystparticle, thereby anchoring the catalyst particle to the substrate.Chemically bonding the catalyst particle to the substrate surfacethrough the control agent helps to keep the catalyst active over time byreducing the tendency of the catalyst particles to agglomerate.

B. Solvents and Carriers

A solvent or carrier may be used as a vehicle for the catalyst atoms(typically in the form of an ionic salt) and/or the control agent. Thesolvent used to make inventive precursor compositions may be an organicsolvent, water or a combination thereof. Preferred solvents are liquidswith sufficient polarity to dissolve the metal salts which are preferredmeans of introducing the catalytic components to the precursor solution.These preferred solvents include water, methanol, ethanol, normal andisopropanol, acetonitrile, acetone, tetrahydrofuran, ethylene glycol,dimethylformamide, dimethylsulfoxide, methylene chloride, and the like,including mixtures thereof.

The solvent for the precursor solution may be neat solvent, but it ispreferable to use an acidic solution, as acids aid in the dissolution ofthe precursor compounds. The solution may be acidified with any suitableacid, including organic and inorganic acids. Preferred acids are mineralacids such as sulfuric, phosphoric, hydrochloric, and the like, orcombinations thereof. While it is possible to use an acid in a widerange of concentrations in the precursor solution, it is generally onlynecessary to use relatively dilute solutions to accomplish the desiredsolubility enhancement. Moreover, concentrated acid solutions maypresent added hazard and expense. Thus, dilute acid solutions arecurrently preferred.

C. Supports and Support Materials

The catalyst particles of the invention may be dispersed without a solidsupport material. For example, they may be dispersed in a solution,suspension, slurry, emulsion, or other liquid medium. The catalystparticles may also be isolated, for example as a fine powder. However,growing the catalyst particles on a support appears to greatly improvethe ability to obtain catalyst particles having the desired controlledcoordination structure. Without the support, it may not be possible toform catalyst particles and then expose the desired crystal face toyield the desired face exposition.

Accordingly, the preferred mode of the invention is for the catalystparticles to be deposited on and supported by a solid support material.Because the term “intermediate precursor composition” may includewhatever components are present prior to formation of the finalsupported reactive catalyst, it may also include the support. Once aportion of the control agent has been removed to expose a portion of thecatalyst particles so as to make them catalytically active, the supportthen becomes part of the final supported active catalyst.

The solid support material may, preferably, be a particle itself or thesolid support may be an essentially continuous solid surface such as afilm, fiber or rod. The solid support material may be organic orinorganic. It may be chemically inert in the chemical reactionenvironment or the solid support itself may serve a catalytic functioncomplementary to the function of the catalyst particles of the presentinvention. The support material may also play a chemical role in thecatalysis process, for example by modifying the structural, electronic,or chemical properties of the dispersed catalytic particles, or bycontributing additional catalytically active sites which directlyparticipate in the overall catalytic process.

The use of a larger quantity of a solid material to serve as a supportfor a lesser quantity of dispersed catalytic particles is a commonstrategy in catalysis and affords a number of known advantages. Theseadvantages include, but are not limited to, more efficient andeconomical use of the catalytic active component, greater stability ofsmall catalytic particles, and separation and spacing of the catalyticparticles to prevent inter-particle effects.

Any solid support material known to those skilled in the art as usefulcatalytic supports may be used as supports for the dispersed controlledcoordination structure catalytic particles of this invention. Thesesupports may be in a variety of physical forms. They may be eitherporous or non-porous. They may be 3-dimensional structures such as apowder, granule, tablet, extrudates, or other 3-dimensional structure.Other 3-dimensional structures include so-called “structured” materials,such as structured packing, which may be in the form of numerousindividual pieces of controlled shape such as rings, saddles, or othershapes, or may be in the form of larger structures, examples of whichinclude structured packings commonly used for distillation and otherphase contacting equipment which involve the use of regular geometricarrangements of convoluted surfaces. Supports may also be in the form ofmainly spherical particles (i.e., beads). Supports may also be in theform of 2-dimensional structures such as films, membranes, coatings, orother mainly 2-dimensional structures.

A variety of material components, alone or in combination, can comprisethe support. One important class of support materials which is preferredfor some applications is porous inorganic materials. These include, butare not limited to, alumina, silica, titania, kieselguhr, diatomaceousearth, bentonite, clay, zirconia, magnesia, as well as the oxides ofvarious other metals, alone or in combination. They also include theclass of porous solids collectively known as zeolites, natural orsynthetic, which have ordered porous structures.

Another useful class of supports preferred for some applications includecarbon-based materials, such as carbon black, activated carbon,graphite, fluoridated carbon, and the like.

Another useful class of support materials include organic solids, suchas polymers. These can include polymer membranes such as those used forthe electrodes of fuel cells. They can also include polymeric orresinous beads, such as those commonly used as ion exchange resins andpolymeric adsorbents.

Another useful class of support materials include metals and metalalloys.

In the case when the catalyst particles of the subject invention areattached to a support as part of a final supported reactive catalyst,the catalyst particles may be deposited in a wide range of loadings onthe support material, ranging from 0.01% to 75% by weight of the totalweight of the supported reactive catalyst, with a preferred range of0.1% to 25% by total weight of the supported reactive catalyst. However,when the primary catalytic component of the dispersed particlescomprises one or more noble metals, it is preferred that the loading ofnoble metal be relatively low so as to economize the expensive activemetal and prevent interparticle interactions, agglomeration, sintering,and other undesirable phenomena. In these cases the preferred catalystload will be within a range from about 0.01% to about 10% by weight ofthe supported reactive catalyst, more preferably in a range of about0.1% to about 5% by weight of the supported reactive catalyst. It shouldbe noted that even when noble metals are used, other components may alsobe added to the catalyst particles, and may be added at higher loadings,such that the total weight of catalyst particles can comprise as much as75% of the total weight of the supported reactive catalyst.

In the case where porous solids are used as the support material, it ispreferred that the surface area of the support be at least 20 m²/g, andmore preferably more than 50 m²/g.

IV. Methods of Making and Using Intermediate Precursor Compositions andSupported Reactive Catalysts

The process for manufacturing intermediate precursor compositions andsupported reactive catalysts therefrom can be broadly summarized asfollows. First, one or more types of catalyst atoms and one or moretypes of control agents are selected. Second, the catalyst atoms andcontrol agent are reacted or combined together to form a catalystcomplex, generally by first dissolving the catalyst atoms and controlagent in an appropriate solvent or carrier in the form of ionic saltsand then allowing the salts to recombine as the catalyst complex so asto form a solution or colloidal suspension. Third, the catalyst complexis applied to a support material to form an intermediate supportedcatalyst complex. Fourth, a portion of the control agent is removed toexpose at least the top or outer layer of catalyst atoms so as to formactive catalyst particles, while a portion of the control agent remainsat the interface between the support and catalyst particles to assist inanchoring the catalyst particles to the support. At some point along theway, the control agent may form a chemical bond with the support.

An exemplary catalyst complex between a catalyst metal (i.e., palladium)and a control agent is schematically illustrated in FIG. 4. Removing aportion of the control agent yields a catalyst particle that may bechemically anchored to the support material by the remaining portion ofthe control agent, as schematically illustrated in FIG. 5. As showntherein, a supported catalyst 10 includes a support 12, which initiallyincludes hydroxyl groups on a surface thereof, an anchoring agent 14chemically bonded to the hydroxyl groups of the support 12 by acondensation reaction, and a catalyst particle 16 bonded or attached insome manner (not shown) to the anchoring agent. When the control agentis removed by hydrogenation or other processes disclosed below, it istheorized that the catalyst atoms collapse or coalesce together to formstructured crystals or particles.

In view of the foregoing, it is apparent that every step before thefinal supported reactive catalyst is formed involves the formation oruse of an intermediate precursor composition in one form or another. Inone aspect of the invention, the “intermediate precursor composition”may be considered to be the catalyst complex comprising the catalystatoms and control agent, exclusive of the surrounding solvent orcarrier. Indeed, it is within the scope of the invention to create acatalyst complex in solution, or as a colloid or suspension, and thenremove the solvent or carrier so as to yield a dried catalyst complexthat can be later added to an appropriate solvent or carrier toreconstitute a solution or colloidal suspension containing the catalystcomplex. Thus, in another aspect of the invention, an “intermediateprecursor composition” according to the invention may include one ormore different solvents or carriers into which the catalyst complex maybe dispersed. The catalyst complex may be applied, or even bonded, to asupport prior to removing a portion of the control agent so as to exposea portion of the catalyst atoms to reveal the desired controlledcoordination structure. Thus, an “intermediate precursor composition”according to the invention may include the catalyst complex and asupport, with or without a solvent or carrier. Accordingly, anycomposition that includes a catalyst complex comprising catalyst atomscomplexed with a control agent in an ordered array (as schematicallyillustrated in FIG. 4) may be considered to comprise an “intermediateprecursor composition”.

Exemplary methods for making catalysts according to the inventioninclude providing one or more types of catalyst atoms in solution (e.g.,in the form of an ionic salt), providing a control agent in solution(e.g., in the form of a carboxylic acid salt), and reacting the catalystatoms with the control agent to form a precursor composition comprisinga complex of the catalytic component and the control agent. The finedispersion of the catalytic component within an appropriate solvent orcarrier by the control agent may be colloidal.

The catalyst atoms can be provided in any form so as to be soluble ordispersible in the solvent or carrier that is used to form the catalystcomplex. In the case where the catalyst atoms comprise one or moremetals, salts of these metals can be formed that are readily soluble inthe solvent or carrier. In the case where the catalyst atoms includenoble metals, it is advantageous to use noble metal chlorides andnitrates, since chlorides and nitrate of noble metals are more readilysoluble than other salts. Chlorides and nitrates of other metal catalystatoms, such as base transition metals and rare earth metals may likewisebe used since chlorides and nitrates are typically more soluble thanother types of salts.

These catalyst atoms can be added to the solvent or carrier singly or incombination to provide final catalyst particles that comprise mixturesof various types of catalyst atoms. For example, a bimetallicpalladium/platinum catalyst can be formed by first forming a precursorsolution in which is dissolved a palladium salt, such as palladiumchloride, and a platinum salt, such as chloroplatinic acid. In general,the composition of the final catalyst will be determined by the types ofcatalyst atoms added to the precursor solution. Therefore, control ofthe amounts of precursor salts added to the solution provides aconvenient method to control the relative concentrations of differenttypes of catalyst atoms in the final catalyst particles.

The control agent is added to the solvent or carrier in a manner so asto facilitate association with the catalyst atoms in order to form thecatalyst complex. Some control agents may themselves be soluble in thesolvent or carrier. In the case of control agents that includecarboxylic acid groups, it may be advantageous to form a metal salt ofthe acids (e.g., an alkali or alkaline earth metal salt). For example,polyacrylic acid can be provided as a sodium polyacrylate salt, which isboth readily soluble in aqueous solvent systems and able to react withcatalyst metal salts to form a catalyst metal-polyacrylate complex,which may be soluble or which may form a colloidal suspension within thesolvent or carrier.

One aspect of the invention is that very small catalytic particles canbe controllably formed (e.g., less than about 100 nm, preferably lessthan about 10 nm, more preferably less than 6 nm, and most preferablyless than 4 nm). The inventors believe that the use of an excessquantity of the control agent plays a factor in determining the size ofthe resulting catalyst particles.

In the case where the catalyst particles of the invention are to beformed on a solid support material, the intermediate precursorcomposition comprising the catalyst complex between the catalyst atomsand control agent is physically contacted with the solid support.Contacting the catalyst complex with the solid support is typicallyaccomplished by means of an appropriate solvent or carrier within theintermediate precursor composition in order to apply or impregnate thecatalyst complex onto the support surface.

Depending on the physical form of the solid support, the process ofcontacting or applying the catalyst complex to the support may beaccomplished by a variety of methods. For example, the support may besubmerged or dipped into a solution or suspension comprising a solventor carrier and the catalyst complex. Alternatively, the solution orsuspension may be sprayed, poured, painted, or otherwise applied to thesupport. Thereafter, the solvent or carrier is removed, optionally inconnection with a reaction step that causes the control agent to becomechemically bonded or adhered to the support. This yields a supportedcatalyst complex in which the active catalyst atoms are arranged in adesired fashion, both in terms of their special orientation resultingfrom the control agent and the arrangement of the control agent on thesupport.

In order to expose at least a portion of catalyst atoms and yield anactive supported catalyst, a portion of the control agent is removed,such as by reduction (e.g., hydrogenation) or oxidation. Hydrogen is thepreferred reducing agent. Instead of or in addition to using hydrogen asthe reducing agent, a variety of other reducing agents may be used,including lithium aluminum hydride, sodium hydride, sodium borohydride,sodium bisulfite, sodium thiosulfate, hydroquinone, methanol, andaldehydes, and the like. The reduction process may be conducted at atemperature between 20° C. and 500° C., and preferably between 100° C.and 400° C. It should be pointed out that oxidation is more suitablewhen the catalyst atoms do not include noble metals, since noble metalcatalysts might catalyze the oxidation of the entire control agent,leaving none for anchoring. Oxidation is more suitable (e.g., at amaximum temperature of 150° C.), for example, in the case where thecatalyst atoms comprise transition metals and the support isnon-combustible (e.g., silica or alumina rather than carbon black,graphite or polymer membranes).

While not to be construed as limiting the scope of the invention, thefollowing explanation can be offered for the usefulness of the reductionstep. It is believed that as the catalyst particle is forming, much ofthe active surface of the catalyst particles is covered by the controlagent molecules. The reduction step serves to modify this surfacestructure, revealing the desired controlled coordination structure ofthe catalyst particles. This step may remove the control agentmolecules, or relocate or reorient those molecules on the catalystsurface, or some combination thereof.

The process of removing the control agent to expose the catalyst atomsis carefully controlled to ensure that enough of the control agentremains so as to reliably anchor the catalyst particles to the support.Thus, at least that portion of the control agent interposed between thesupport and the bottom surface of the catalyst particles facing thesupport is advantageously left intact. It is theorized that duringremoval of the control agent (by hydrogenation/reduction,reflux/boiling, or other process), the control agent on the outersurfaces of the catalyst particle is more easily removed than thecontrol agent bonded between the support and the catalyst particle.Thus, the remaining control agent disposed between the support and thecatalytic particle acts to anchor the catalytic particle to the support.This results in catalyst particles that have enhanced stability withrespect to surface migration, agglomeration, and sintering. On the otherhand, removing the control agent to the extent that little or none of itremains to anchor the catalyst particles to the support has been foundto reduce the long-term stability of the supported catalyst.

The resulting supported reactive catalyst can be optionally heat-treatedto further activate the catalyst. It has been found that, in some cases,subjecting the supported reactive catalyst to a heat treatment processbefore initially using the supported catalyst causes the catalyst to bemore active initially. The step of heat treating the supported catalystmay be referred to as “calcining” because it may act to volatilizecertain components within the supported catalyst. However, it is notcarried out at temperatures high enough to char or destroy the anchoringagent. The heat treatment process may be carried in inert, oxidizing, orreducing atmospheres, but preferably in an inert atmosphere. Where thesupported catalyst is subjected to a heat treatment process, the processis preferably carried out at a temperature in a range of about 50° C. toabout 300° C., more preferably in a range of about 100° C. to about 250°C., and most preferably in a range of about 125° C. to about 200° C. Theduration of the heat treatment process is preferably in a range of about30 minutes to about 12 hours, more preferably in a range of about 1 hourto about 5 hours.

As discussed above, the providing a catalyst having a controlledcoordination number of 2 for atoms on the top or outer layer of reactivecatalyst atoms is useful for a variety of chemical processes. One ofthese involves the direct reaction of hydrogen and oxygen to formhydrogen peroxide. Accordingly, this aspect of the invention is furtherdescribed with the aid of the following examples, which should not beconstrued as limiting the scope of the invention.

EXAMPLE 1

Preparation of Pd/Pt Controlled Coordination Catalyst on Carbon Support

This example describes the preparation of a noble metal catalyst on acarbon support having a top or outer layer of noble metal atoms withcontrolled coordination of 2. The active noble metal constituentincluded a mixture of palladium and platinum. The catalyst support wascarbon black.

A first solution was prepared by dissolving 1.333 g of palladiumchloride in 1000 ml of an acidic aqueous solution that included 0.15%hydrochloric acid. A second solution was prepared by dissolving 15 g ofa 45% sodium polyacrylate solution in 100 ml of water. The sodiumpolyacrylate had a molecular weight of 1200. This batch of sodiumpolyacrylate was believed to include about 80–90% straight-chainedmolecules. A third solution was then prepared by dissolving 0.2614 g ofplatinum chloride in 1000 ml of water. Thereafter, 300 ml of the firstsolution, 40 ml of the second solution, and 48 ml of the third solutionwere mixed together. The combined solution was then diluted up to atotal volume of 4000 ml with water.

The diluted combined solution was then purged with a continuous flow ofnitrogen for 1 hour, and then reduced by a continuous flow of hydrogenfor 20 minutes. The combined solution mixture was designated the“catalyst precursor solution”.

The catalyst precursor solution was then mixed with 24 g of carbon blackhaving a surface area of 200 m²/g. The precursor solution/carbon blackmixture was mixed for 17 hours to ensure thorough impregnation of thesupport by the catalyst precursor solution. The impregnated carbon blackwas then dried overnight to yield an intermediate precursor compositioncomprising a catalyst complex of palladium, platinum and polyacrylateapplied to the carbon black support.

After drying, the intermediate precursor composition was reduced under acontinuous hydrogen flow at 300° C. for 17 hours. After this process wascomplete, an active controlled coordination catalyst was obtained thatis an example of a reactive supported catalyst according to theinvention. The reactive supported catalyst had a noble metal loading of0.7 wt %.

The presence of the desired controlled coordination structure wasestablished by use of high resolution transmission electron microscopy,as shown in FIG. 6. In this micrograph, the carbon support material isvisible as the lighter colored matrix, while, the noble metalcrystallites are visible as darker colored spots. The low magnificationpanels show a uniform dispersion of small (<5 nm) noble metalcrystallites. While a lack of contrast between support and noble metalparticles somewhat hinders the interpretation of the image, thecontrolled coordination structure is evident in the magnified image of asingle noble metal particle shown in the upper left panel of FIG. 6. Aseries of lines visible on the surface of the particle are at atomicdimensions, and represent a direct image of the atomic structure of thesurface. The structure is direct evidence of surface atoms that arecoordinated with only two nearest neighbor top or outer layer atoms. Allother top or outer layer atoms are at greater spacing.

Testing of the supported catalyst, including IR spectroscopy, indicatedthat some of the control agent appeared to remain even after thereduction step. In particular, IR spectroscopy showed that a substantialportion of the hydroxyl groups originally present on the support were nolonger present, suggesting that they had reacted with the control agent.IR spectroscopy also indicated the presence of C—H groups, suggestingthat a portion of the control agent remained after hydrogenation. Thefact that the catalyst particles were much less mobile compared tosupported catalysts that did not include any control agent suggestedthat the remaining control agent acted to anchor the catalyst particlesto the support.

EXAMPLE 2

Preparation of Pd/Pt Controlled Coordination Catalyst on TS-1 Support

This example describes the preparation of a noble metal catalyst on azeolitic support having top or outer layer noble metal atoms withcontrolled coordination of 2. The active noble metal constituentincluded a mixture of palladium and platinum. The catalyst support wastitanium silicalite-1 (TS-1).

A first solution was prepared by dissolving 1.3339 g of palladiumchloride in 1000 ml of an acidic aqueous solution that included 0.15%hydrochloric acid. A second solution was prepared by dissolving 15 g ofa 45% sodium polyacrylate solution in 100 ml of water. The sodiumpolyacrylate had a molecular weight of 1200. A third solution was thenprepared by dissolving 0.2614 g of platinum chloride in 1000 ml ofwater. Thereafter, 75 ml of the first solution, 10 ml of the secondsolution, and 12 ml of the third solution were mixed together. Thecombined solution was then diluted with water up to a total volume of500 ml.

The diluted combined solution was then purged with a continuous flow ofnitrogen for 1 hour, and then reduced by a continuous flow of hydrogenfor 20 minutes to form the catalyst precursor solution.

The catalyst precursor solution was then mixed with 6 g of titaniumsilicalite-1 (TS-1) having a surface area of 370 m²/g. The precursorsolution/TS-1 mixture was mixed for 17 hours to ensure thoroughimpregnation of the support by the precursor solution. The impregnatedTS-1 was then dried overnight. After drying, the impregnated TS-1 wasreduced under continuous hydrogen flow at 300° C. for 17 hours. Afterreduction, an active controlled coordination catalyst was obtained witha noble metal loading of 0.8 wt %.

Again, the presence of the desired controlled coordination surfacestructure was confirmed by use of high resolution transmission electronmicroscopy. In a TEM image shown in FIG. 7, the noble metal particlesare readily visible as darker patches against the lighter supportbackground. Somewhat better imaging contrast is achieved compared toExample 1, so the controlled coordination structure is more easilyvisible in the noble metal particles. The particle surfaces show linesat the atomic scale, indicating that individual surface atoms are onlycoordinated with two adjacent nearest neighbor atoms in the top or outerlayer.

The supported reactive catalysts disclosed above are useful in variousreactions including formation of hydrogen peroxide as a raw material oras an intermediate in the oxidation of organic molecules, directoxidation of organic molecules, hydrogenation, reduction,dehydrogenation, and, potentially, to make fuel cells, which reactionsare summarized below. These reactions can be further optimized byoptimizing solvents, pH, and other reaction conditions.

The hydrogen peroxide made using catalysts of this invention can berecovered as a product in selectivities of 95% to 100%. Alternately, thehydrogen peroxide produced can be used as an intermediate for theproduction of other chemical products. Examples of useful chemicalreactions which may be conducted using the hydrogen peroxideintermediate are the conversion of olefins such as propylene to epoxidesincluding propylene oxide; the conversion of aromatics such as benzeneto hydroxylated aromatics including phenol; and the conversion of acidssuch as acetic acid into peracids including peracetic acid.

The hydrogen peroxide intermediate may be used ex situ or in situ. By exsitu is meant the case where the hydrogen peroxide intermediate iswithdrawn from the hydrogen peroxide synthesis reactor as anintermediate product, and then passed to downstream processing stepswhere it utilized as a reactant in the formation of the desired chemicalproduct. By in situ is meant the case where the hydrogen peroxideintermediate formed by the catalyst of this invention is converted,immediately upon formation and in the same chemical reactor, in a secondchemical reaction to form the desired product. This in situ conversionmay be accomplished using a second, physically separate catalyst for thesecond step reaction. Alternatively, the second-step catalyst may bephysically integrated with the catalyst of the invention into adual-functional catalyst by using the second-step catalyst as thesubstrate for the deposition of the dispersed controlled coordinationcatalyst particles of this invention. In yet another alternative, thesecond step reaction may be non-catalytic.

EXAMPLE 3

Batch Synthesis of Hydrogen Peroxide Using Controlled CoordinationCatalyst

0.2 g of supported catalyst prepared according to Example 1 was chargedto a semi-batch stirred tank reactor with nominal liquid volume of 200ml. 200 ml of liquid solution consisting of water with 1 wt % H₂SO₄ and5 ppmw NaBr was also charged to the reactor. A gas feed consisting of 3vol % hydrogen, 20 vol % oxygen, and 77 vol % nitrogen was fedcontinuously to the reactor at a rate of 1000 sccm. Unreacted gases werecontinuously withdrawn from the reactor. The reactor was maintained at atemperature of 45° C. and a pressure of 1400 psig. A mechanical stirrerwas used to agitate the reaction medium.

Continuous flow of gas was maintained for a period of 3 hours. At theend of the 3 hour run, the gas feed was stopped, and the final liquidremoved from the reactor. Based on gas analysis, the overall averagehydrogen conversion was found to be 33%. Liquid analysis showed a finalhydrogen peroxide concentration of 4.8 wt %. The hydrogen peroxideselectivity with respect to hydrogen converted was found to be 100%.

EXAMPLE 4

Continuous Synthesis of Hydrogen Peroxide Using Controlled CoordinationCatalyst

0.837 g of supported catalyst prepared according to Example 1 wascharged to a continuous stirred tank reactor (CSTR) with nominal liquidvolume of 200 ml. A liquid solution consisting of methanol with 1 wt %H₂SO₄ and 5 ppmw NaBr was continuously fed to the reactor at a rate of100 cc/hr. A gas feed consisting of 3 vol % hydrogen and 97 vol % oxygenwas fed to the reactor at a rate of 5200 sccm. The reactor wasmaintained at a temperature of 35° C. and a pressure of 1400 psig. Amechanical stirrer was used to agitate the reaction medium. An internalfilter was attached to the reactor outlet connection to allow gas andliquid products to be continuously withdrawn from the reactor whilemaintaining the catalyst within the reactor.

The reactor was maintained in this continuous operation for a period of150 hours. After allowing for an initial lineout period of about 30hours, a period of steady-state operation was observed for a period of120 hours. During this period, the average conversion of hydrogen wasfound to be 42%. The average selectivity of hydrogen peroxide formation,based on hydrogen converted, was found to be 95%. The average liquidproduct hydrogen peroxide concentration was found to be 6.8 wt %.

EXAMPLES 5–12

Heat Treatment of Supported Catalyst to Increase CatalyticActivity/Selectivity

Eight catalyst samples were prepared. The following procedure was commonto all eight catalysts. A palladium salt solution was prepared by mixing1.3339 g PdCl₂ with 4 ml HCl and 996 ml de-ionized water. The resultingsolution contained 0.0799 wt. % (7.511×10⁻³ M) palladium. A platinumsalt solution was prepared by mixing 0.2614 g H₂PtCl₆ with 1000 mlde-ionized water. The resulting solution contained 0.010 wt. %(5.126×10⁻⁴ M) platinum. A templating agent solution was prepared bydiluting 16 g of 45 wt. % polyacrylic acid solution (MW approximately1200 Daltons) to a total weight of 100 g with de-ionized water. Theresulting solution contained 6.75 wt. % polyacrylic acid.

In order to prepare 48 grams of 1% Pd+0.02% Pt/C catalyst, 600 ml of thepalladium salt solution was mixed with 96 ml of the platinum saltsolution and 80 ml of the polyacrylic acid solution. The resultingmixture was diluted to 8000 ml with de-ionized water. The dilutedsolution was purged with 100 ml/min N₂ for 1 hour. The N₂ was thenreplaced by 100 ml/min H₂ for 20 minutes. 48 g of carbon black (BP-700from Cabot) was added to the diluted solution. The resulting mixture wasthen boiled to remove most of the liquid, followed by drying to obtain adry solid.

The dry solid was then reduced as follows:

-   -   1. Purged with 100 ml/min N₂ for 30 minutes;    -   2. Switched to 100 ml/min H₂;    -   3. Temperature ramped from 30° C. to 90° C. over 30 minutes;    -   4. Temperature held at 90° C. for 2 hours;    -   5. Temperature ramped from 90° C. to 300° C. over 2 hours;    -   6. Temperature held at 300° C. for 17 hours.

Different catalyst samples were then subjected to a heat treatmentprocess under an inert N₂ atmosphere at various temperatures for 3 hoursaccording to the following table:

Example Heat Treatment 5 None 6 100° C. 7 125° C. 8 150° C. 9 175° C.10  200° C. 11  225° C. 12  250° C.

The supported catalysts made according of Examples 5–12 were used toconduct a semi-batch synthesis of hydrogen peroxide from hydrogen andoxygen in a stirred reactor. For each batch, 75 g of liquid feed mixturewas used consisting of water with 1% H₂SO₄ and pp, NaBr. 0.25 g ofcatalyst was added according to the table below. The synthesis reactionwas conducted at 45° C. and 1000 psig with a continuous feed of 1000sccm of gas containing 3.15% hydrogen in air. The outlet gas wasanalyzed by CG to determine the extent of hydrogen conversion. Thereaction was run for 2 hours, at which time the feed gas was stopped andthe liquid product removed and analyzed for hydrogen peroxide content.The results were as follows, where the hydrogen peroxide selectivity iscalculated as moles of hydrogen peroxide produced per moles of hydrogenconsumed.

H₂O₂ Conc. Hydrogen H₂O₂ Example (wt %) Conversion (%) Selectivity (%) 50.39% 12.92% 41.0% 6 2.312% 38.60% 86.75% 7 2.515% 39.82% 90.79% 8 2.46%40.23% 89.13% 9 2.437% 39.54% 91.03% 10  2.31% 38.16% 88.0% 11  2.244%36.91% 88.33% 12  1.961% 33.29% 84.89%

The foregoing data should not be interpreted to mean that supportedcatalysts according to the invention that are not subjected to a heattreatment lack high specificity. In fact, selectivity for non-heattreated catalysts increased over time and was high following an initialbreak-in period. The heat treatment process appears to accelerateselectivity, perhaps also increasing it over time.

The supported reactive catalysts of this invention are also useful forreactions of oxygen and organic compounds to form oxidized chemicalproducts (direct oxidation). A variety of chemical substrates can beoxidized to form useful products. A list of examples, not to beconstrued as limiting the scope of the invention, includes: (a) thedirect oxidation of olefins to produce acids, such propylene oxidationto acrylic acid or ethylene oxidation to acetic acid; (b) directepoxidation of olefins to form epoxides, for example propylene topropylene oxide or ethylene to ethylene oxide; (c) the oxidation ofxylenes to phthalic acids or related compounds, for example p-xylene toterephthallic acid, o-xylene to phthalic anhydride, and m-xylene toisophthalic acid; (d) the oxidation of ethylene to vinyl acetate; (e)the oxidation of ethylene to acetaldehyde; and (f) the oxidation ofisobutylene to methacrylic acid. The preferred choice of activecomponent for these catalysts will depend on the specific application.For example, the preferred main active component for the conversion ofolefins to carboxylic acids is palladium, while the preferred activecomponent for the conversion of olefins to epoxides is silver.

The supported reactive catalysts of this invention are also useful forhydrogenation reactions. The catalyst is particularly advantageous incases where selectivity is a critical factor, i.e., cases where thefeedstock contains more than one reducible functional group, but onlycertain of these functional groups should be hydrogenated. A particularexample, not meant to be limiting of the scope of the invention, isuseful in illustrating the utility of the invention. In the selectivehydrogenation of acetophenone to methyl benzyl alcohol, the catalystshould selectively hydrogenate the carbonyl group, while leaving thebenzene ring of the acetophenone molecule unaffected. A preferredcatalyst for this application will be based on a major active componentof platinum, palladium, or ruthenium. On the catalytic particles basedon this invention, the double bond of the carbonyl group can be readilyadsorbed in alignment with the top row of surface atoms on the catalystsurface, thereby positioning this bond for attack by hydrogen fromadjacent surface sites. However, the bulkier aromatic ring does not fiton the linear surface structure, and will not be readily hydrogenated.

The catalyst of this invention is also useful for other reductivereactions, including but not limited to ammonia synthesis,carbonylation, hydroformylation, oil and fat hardening, reductivealkylation, reductive amination, and hydrosilation.

The catalyst of this invention is also useful for reactions whichliberate hydrogen from reactant molecules, including dehydrogenation andreforming. For catalytic reforming of petroleum fractions and otherhydrocarbons to form aromatic compounds, the preferred active catalyticcomponent is platinum or a combination of platinum and rhenium. Forreforming of hydrocarbons to form syngas, useful catalytic componentsare noble metals such as palladium, platinum, iridium, gold, osmium,ruthenium, rhodium, rhenium, and combinations thereof. The same noblemetals, either singly or in combination, are useful for dehydrogenationreactions such as the conversion of propane to propylene or ethane toethylene.

The catalyst of this invention is also useful as the catalytic componentin a fuel cell electrode for polymer electrolyte membrane fuel cells anddirect methanol fuel cells. On both the anode and cathode of these fuelscells, a controlled coordination catalyst based on platinum as the majoractive component is useful.

The present invention may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. The describedembodiments are to be considered in all respects only as illustrativeand not restrictive. The scope of the invention is, therefore, indicatedby the appended claims rather than by the foregoing description. Allchanges which come within the meaning and range of equivalency of theclaims are to be embraced within their scope.

1. An intermediate precursor composition for use in manufacturing asupported catalyst having a controlled coordination structure,comprising: a plurality of catalyst atoms comprising at least one memberselected from the group comprising noble metals, rare earth metals, andtransition metals, and optionally one or more non-metals; and a controlagent comprising a plurality of complexing molecules selected from thegroup comprising polymers, oligomers, and organic compounds, eachcomplexing molecule having a plurality of functional groups disposedalong a backbone for complexing the catalyst atoms to the complexingmolecules, at least about 50% of the complexing molecules beingstraight-chained molecules that include at least four functional groupsper molecule, and at least a portion of the complexing molecules forminga catalyst complex between the catalyst atoms and the complexingmolecules, wherein the catalyst complex, after being applied to asupport, forms a supported reactive catalyst comprising a support and aplurality of reactive catalyst particles formed from the catalystcomplex, wherein the complexing molecules and catalyst atoms interact insuch a way that a preponderance of catalyst atoms on an upper surface ofsupported reactive catalyst particles formed from the catalyst complexwill have a nearest neighbor coordination number of
 2. 2. Anintermediate precursor composition as defined in claim 1, wherein atleast a portion of the catalyst atoms comprise at least one noble metalselected from the group comprising palladium, platinum, iridium, gold,osmium, ruthenium, rhodium, and rhenium.
 3. An intermediate precursorcomposition as defined in claim 1, wherein at least a portion of thecatalyst atoms comprise at least one transition metal.
 4. Anintermediate precursor composition as defined in claim 3, wherein thetransition metal comprises at least one member selected from the groupcomprising chromium, manganese, iron, cobalt, nickel, copper, zirconium,tin, zinc, tungsten, titanium, molybdenum, and vanadium.
 5. Anintermediate precursor composition as defined in claim 1, wherein atleast a portion of the catalyst atoms comprise at least one rare earthmetal.
 6. An intermediate precursor composition as defined in claim 5,wherein the rare earth metal comprises at least one member selected fromthe group comprising lanthanum and cerium.
 7. An intermediate precursorcomposition as defined in claim 1, wherein a portion of the catalystatoms comprise at least one non-metal.
 8. An intermediate precursorcomposition as defined in claim 1, further comprising at least one of analkali metal or alkaline earth metal.
 9. An intermediate precursorcomposition as defined in claim 1, wherein at least a portion of thefunctional groups comprise a carbon atom bonded to at least oneelectron-rich atom that is more electronegative than the carbon atom andthat is able to donate one or more electrons so as to form a bond orattraction with at least one of the catalyst atoms.
 10. An intermediateprecursor composition as defined in claim 9, wherein the electron-richatom comprises at least one of oxygen or nitrogen.
 11. An intermediateprecursor composition as defined in claim 9, wherein the electron-richatom has a negative charge and the catalyst atoms have a positivecharge.
 12. An intermediate precursor composition as defined in claim 1,wherein at least about 60% of the complexing molecules arestraight-chained.
 13. An intermediate precursor composition as definedin claim 1, wherein at least about 75% of the complexing molecules arestraight-chained.
 14. An intermediate precursor composition as definedin claim 1, wherein at least about 90% of the complexing molecules arestraight-chained.
 15. An intermediate precursor composition as definedin claim 1, wherein at least about 95% of the complexing molecules arestraight-chained.
 16. An intermediate precursor composition as definedin claim 1, wherein about 100% of the complexing molecules arestraight-chained.
 17. An intermediate precursor composition as definedin claim 1, further comprising a solvent or carrier into which thecatalyst complex and any remaining catalyst atoms and control agent aremixed.
 18. An intermediate precursor composition as defined in claim 17,wherein the solvent or carrier comprises water.
 19. An intermediateprecursor composition as defined in claim 17, wherein the solvent orcarrier comprises at least one aqueous acid.
 20. An intermediateprecursor composition as defined in claim 17, wherein the solvent orcarrier comprises at least one organic solvent.
 21. An intermediateprecursor composition as defined in claim 17, further comprising atleast one support material.
 22. An intermediate precursor composition asdefined in claim 21, wherein the catalyst complex is impregnated within,but not chemically bonded to, the support material.
 23. An intermediateprecursor composition as defined in claim 21, wherein the catalystcomplex is chemically bonded to the support material.
 24. Anintermediate precursor composition as defined in claim 1, furthercomprising at least one support material to which the catalyst complexis chemically bonded.
 25. An intermediate precursor composition asdefined in claim 1, wherein the control agent has a number averagemolecular weight in a range of about 300 to about 15,000 Daltons.
 26. Anintermediate precursor composition as defined in claim 1, wherein thecontrol agent has a number average molecular weight in a range of about600 to about 6,000 Daltons.
 27. An intermediate precursor composition asdefined in claim 1, wherein a substantial portion of the control agentincludes from about 4 to about 200 of the functional groups percomplexing molecule.
 28. An intermediate precursor composition asdefined in claim 1, wherein a substantial portion or the control agentincludes from about 8 to about 80 functional groups per complexingmolecule.
 29. An intermediate precursor composition as defined in claim1, wherein a substantial portion of the control agent includes fromabout 10 to about 20 functional groups per complexing molecule.
 30. Anintermediate precursor composition as defined in claim 1, wherein theintermediate precursor composition includes a molar ratio of controlagent functional groups to catalyst atoms in a range of about 0.5:1 toabout 40:1.
 31. An intermediate precursor composition as defined inclaim 1, wherein the intermediate precursor composition includes a molarratio of control agent functional groups to catalyst atoms in a range ofabout 1:1 to about 35:1.
 32. An intermediate precursor composition asdefined in claim 1, wherein the intermediate precursor compositionincludes a molar ratio of control agent functional groups to catalystatoms in a range of about 3:1 to about 30:1.
 33. An intermediateprecursor composition as defined in claim 1, wherein the control agentcomprises at least one of polacrylic acid or a polyacrylic acid salt.34. An intermediate precursor composition as defined in claim 1, whereinthe control agent comprises at least one member selected from the groupcomprising polyvinylbenzoates, polyvinyl sulfate, polyvinyl sulfonatesincluding sulfonated styrene, polybisphenol carbonates,polybenzimidizoles, polypyridine, sulfonated polyethylene terephthalate,polyvinyl alcohol, polyethylene glycol, and polypropylene glycol.
 35. Anintermediate precursor composition as defined in claim 1, wherein thecatalyst complex comprises a random distribution of at least twodifferent types of catalyst atoms.
 36. An intermediate precursorcomposition as defined in claim 35, wherein the catalyst complex forms asupported reactive catalyst in a manner so that reactive catalystparticles formed from the catalyst complex will also include a randomdistribution of at least two different types of catalyst atoms.
 37. Anintermediate precursor composition for use in manufacturing a supportedcatalyst having a controlled coordination structure, comprising: aplurality of catalyst atoms comprising at least one member selected fromthe group comprising noble metals, rare earth metals, and transitionmetals, and optionally one or more non-metals; and a control agentcomprising a plurality of complexing molecules selected from the groupcomprising polymers and oligomers, each complexing molecule having aplurality of functional groups disposed along a backbone for complexingthe catalyst atoms to the complexing molecules, at least about 50% ofthe complexing molecules being straight-chained and at least a portionof the complexing molecules forming a catalyst complex between thecatalyst atoms and the complexing molecules, wherein the catalystcomplex, after being applied to a support, forms a supported reactivecatalyst comprising a support and a plurality of reactive catalystparticles formed from the catalyst complex, wherein the complexingmolecules and catalyst atoms interact in such a way that a preponderanceof catalyst atoms on an upper surface of supported reactive catalystparticles formed from the catalyst complex will have a nearest neighborcoordination number of
 2. 38. An intermediate precursor composition asdefined in claim 37, wherein at least about 75% of the complexingmolecules are straight-chained.
 39. An intermediate precursorcomposition as defined in claim 37, wherein at least about 90% of thecomplexing molecules are straight-chained.
 40. An intermediate precursorcomposition as defined in claim 37, further comprising a solvent orcarrier into which the catalyst complex and any remaining catalyst atomsand control agent are dissolved or dispersed.
 41. An intermediateprecursor composition as defined in claim 40, further comprising atleast one support material.
 42. An intermediate precursor composition asdefined in claim 41, wherein the catalyst complex is impregnated within,but not chemically bonded to, the support material.
 43. An intermediateprecursor composition as defined in claim 41, wherein the catalystcomplex is chemically bonded to the support material.
 44. Anintermediate precursor composition as defined in claim 37, furthercomprising at least one support material to which the catalyst complexis chemically bonded.
 45. An intermediate precursor composition asdefined in claim 37, wherein the control agent comprises at least one ofpolacrylic acid or a polyacrylic acid salt.
 46. An intermediateprecursor composition as defined in claim 37, wherein the control agentcomprises at least one member selected from the group comprisingpolyvinylbenzoates, polyvinyl sulfate, polyvinyl sulfonates includingsulfonated styrene, polybisphenol carbonates, polybenzimidizoles,polypyridine, sulfonated polyethylene terephthalate, polyvinyl alcohol,polyethylene glycol, and polypropylene glycol.
 47. An intermediateprecursor composition as defined in claim 37, wherein the catalystcomplex comprises a random distribution of at least two different typesof catalyst atoms.
 48. An intermediate precursor composition as definedin claim 47, wherein the catalyst complex forms a supported reactivecatalyst in a manner so that reactive catalyst particles formed from thecatalyst complex will also include a random distribution of at least twodifferent types of catalyst atoms.
 49. A method of preparing anintermediate precursor composition for use in manufacturing a supportedcatalyst having a controlled coordination structure, the methodcomprising: providing a plurality of catalyst atoms comprising at leastmember selected from the group comprising noble metals, rare earthmetals, and transition metals, and optionally one or more non-metals;providing a control agent comprising a plurality of complexing moleculesselected from the group comprising polymers, oligomers, and organiccompounds, each complexing molecule having a plurality of functionalgroups disposed along a backbone for complexing the reactive catalystatoms to the complexing molecules, wherein at least about 50% of thecomplexing molecules are straight-chained molecules that include atleast four of the functional groups per molecule; mixing together thecatalyst atoms and control agent in a liquid to form a mixture; andreacting at least a portion of the catalyst atoms with at least aportion of the control agent to yield a catalyst complex that, afterbeing applied to a support, forms a supported reactive catalystcomprising a support and a plurality of reactive catalyst particlesformed from the catalyst complex, wherein the complexing molecules andcatalyst atoms interact in such a way that a preponderance of catalystatoms on an upper surface of supported reactive catalyst particlesformed from the catalyst complex will have a nearest neighborcoordination number of
 2. 50. A method of preparing an intermediateprecursor composition as defined in claim 49, further comprisingcontacting the catalyst complex with a support.
 51. A method ofpreparing an intermediate precursor composition as defined in claim 50,wherein the catalyst complex is impregnated within, but not bonded to,the support.
 52. A method of preparing an intermediate precursorcomposition as defined in claim 50, further comprising reacting thecatalyst complex with the support so that a portion of the control agentchemically bonds the catalyst complex to the support.
 53. A method ofpreparing an intermediate precursor composition as defined in claim 52,wherein the portion of the control agent that bonds the catalyst complexto the support does so by means of a condensation reaction.
 54. A methodof preparing an intermediate precursor composition as defined in claim52, wherein the portion of the control agent that bonds the catalystcomplex to the support comprises an anchoring agent.
 55. A method ofpreparing an intermediate precursor composition as defined in claim 52,further comprising removing the liquid so as to yield a supportedcatalyst precursor composition comprising the catalyst complex bonded tothe support.
 56. A method of preparing an intermediate precursorcomposition as defined in claim 49, wherein the catalyst complexincludes a random distribution of at least two different types ofcatalyst atoms.
 57. A method of preparing an intermediate precursorcomposition for use in manufacturing a supported catalyst having acontrolled coordination structure, the method comprising: providing aplurality of catalyst atoms comprising at least member selected from thegroup comprising noble metals, rare earth metals, and transition metals,and optionally one or more non-metals; providing a control agentcomprising a plurality of complexing molecules selected from the groupcomprising polymers and oligomers, each control agent molecule having aplurality of functional groups disposed along a backbone for complexingthe reactive catalyst atoms to the complexing molecules, wherein atleast about 50% of the complexing molecules arc straight-chained; mixingtogether the catalyst atoms and control agent in a liquid to form amixture; and reacting at least a portion of the catalyst atoms with atleast a portion of the control agent to yield a catalyst complex that,when applied to a support, forms a supported reactive catalystcomprising a support and a plurality of reactive catalyst particlesformed from the catalyst complex, wherein the complexing molecules andcatalyst atoms interact in such a way that a preponderance of catalystatoms on an upper surface of supported reactive catalyst particlesformed from the catalyst complex will have a nearest neighborcoordination number of
 2. 58. A method of preparing an intermediateprecursor composition as defined in claim 57, further comprisingcontacting the catalyst complex with a support.
 59. A method ofpreparing an intermediate precursor composition as defined in claim 58,wherein the catalyst complex is impregnated within, but not bonded to,the support.
 60. A method of preparing an intermediate precursorcomposition as defined in claim 58, further comprising reacting thecatalyst complex with the support so that a portion of the control agentchemically bonds the catalyst complex to the support.
 61. A method ofpreparing an intermediate precursor composition as defined in claim 60,wherein the portion of the control agent that bonds the catalyst complexto the support does so by means of a condensation reaction.
 62. A methodor preparing an intermediate precursor composition as defined in claim60, wherein the portion of the control agent that bonds the catalystcomplex to the support comprises an anchoring agent.
 63. A method ofpreparing an intermediate precursor composition as defined in claim 60,further comprising removing the liquid so as to yield a supportedcatalyst precursor composition comprising the catalyst complex bonded tothe support.
 64. A method of preparing an intermediate precursorcomposition as defined in claim 57, wherein the catalyst complexincludes a random distribution of at least two different types ofcatalyst atoms.